1. Field of the Invention
This invention relates to an electrostatic recording head used in an electrostatic recording apparatus and a method of making the same. More particularly, this invention relates to an electrostatic recording head having a high dimensional accuracy in the direction of the width of its electrode exposed surface as well as an excellent linearity of the electrodes in the direction of its length, and a method of making the same.
2. Description of the Related Art
An electrostatic recording head of multiple electrode type is used in an electrostatic recording apparatus in which, in response to the application of image signals from a computer, electrographics or like, an electrostatic latent image is formed at a high speed on a recording medium having a charge holding surface and an electrical conductive layer, and a toner is used to develop the electrostatic latent image into a visible image. Such an electrostatic recording head is already known from the disclosures of, for example, JP-A-53-20929, JP-A-56-110959 and JP-A-56-122056. This electrostatic recording head includes a plurality of elongate recording electrodes of an electrical conductive material disposed in parallel to each other on both surfaces of a substrate of an electrical insulating material such as a glass or an epoxy resin. When the electrostatic recording apparatus is used for electrostatic recording of an image on a recording medium, the exposed end surfaces of the electrodes of the recording head and the end surfaces of separately provided auxiliary electrodes are brought into contact with a moving recording medium, signal voltages corresponding to image signals are applied across the recording electrodes and the auxiliary electrodes to form an electrostatic latent image on the recording medium, and then this electrostatic latent image is developed by toner thereby recording a visible image on the recording medium.
The recording electrodes are provided on both surfaces of the substrate of the recording head so as to improve the recording density. However, because of the difficulty of forming many recording electrodes in an accurate positional relationship on both surfaces of a single substrate, it is a common practice that a pair of sub-substrates each having a series of equally spaced recording electrodes formed on its one surface are bonded together by a bonding agent in such a manner than the electrodes on one of the sub-substrates are located, respectively, opposite to the spaces between the adjacent electrodes on the other sub-substrate, thereby obtaining a composite electrode substrate carrying all the recording electrodes needed for the recording head.
FIG. 9 is a schematic perspective view of part of an end surface of such an electrode substrate 50 made in the manner described above. Referring to FIG. 9, the electrode substrate 50 includes a pair of sub-substrates 1 and 2 of an electrical insulator such as a glass or an epoxy resin, and a plurality of electrodes 3 of copper are formed in parallel to each other on one of the surfaces of each of the sub-substrates 1 and 2. The sub-substrates 1 and 2 are bonded together at the other or back surfaces by a bonding agent 4 to be unified into an integral structure.
The electrode substrate 50 shown in FIG. 9 is made by the steps of first depositing a thin film of copper on one surface of each of the sub-substrates 1 and 2 before being bonded, coating portions of the thin copper film corresponding to the electrodes with a photo resist according to the photolithography technique so as to cover these portions, and then etching to remove unnecessary portions of the thin copper film. The resultant sub-substrates 1 and 2 are disposed so that the electrodes on one of the sub-substrates are located, respectively, opposite to the spaces between adjacent electrodes on the other sub-substrate as shown in FIG. 9, and the sub-substrates 1 and 2 are then bonded together by the bonding agent 4 to complete the electrode substrate 50 having the recording electrodes on its both surfaces.
When electrostatic recording is to be carried out by the recording head having such an electrode substrate 50, a recording sheet A is brought into sliding contact with the end surfaces of the sub-substrates 1, 2 and electrodes 3 as shown by the broken lines in FIG. 9, applying image signal voltages across the recording electrodes 3 and associated auxiliary electrodes (not shown) located in the vicinity of the electrodes 3 thereby forming an electrostatic latent image on the recording sheet A, and then this electrostatic latent image is developed by toner thereby recording a visible image on the recording sheet A.
By providing the recording electrodes 3 on both surfaces of the electrode substrate 50 as shown in FIG. 9, the recording density can be improved. That is, after first applying signal voltages to the electrodes 3 on the sub-substrate 2, the recording sheet A is fed in the direction as shown by the arrow by a distance corresponding to the thickness of the electrode substrate 50, and signal voltages ar then applied to the electrodes 3 on the sub-substrate 1. Thus, the recording density that can be achieved is two times as high as that obtained when the electrodes 3 are formed on one surface only of the electrode substrate 50.
A dimensional error in the direction of the thickness of the electrode substrate 50 must be as small as possible. Especially, when three or four recording heads each having the electrode substrate 50 shown in FIG. 9 are arranged in parallel to each other to effect color printing, a large dimensional error in the thicknesswise direction of each of the electrode substrates 50 results in undesirable nonuniformity, blur or the like of the colors of the recorded image, and the image quality is greatly degraded.
However, in the case of the prior art electrode substrate 50, the two sub-substrates 1 and 2 are merely bonded together by the bonding agent 4. Therefore, when the electrode substrate 50 has a large length of, for example, 36 inches, it is very difficult to uniformly apply the bonding agent 4 so as to maintain the thickness of the layer of the bonding agent 4 uniform over the whole length of the sub-substrates 1 and 2 having such a large length. Thus, in the case of the prior art electrode substrate 50, the thickness of the layer of the bonding agent 4 could not be maintained uniform over the whole length, and it has been difficult to maintain the desired uniform dimension in the thicknesswise direction of the electrode substrate 50.
For the purpose of making an electrostatic recording head by the use of the electrode substrate 50 shown in FIG. 9, the electrode substrate 50 is inserted at its front end into a U-shaped groove 54 of a mold 53 as shown in FIG. 10, and a molding resin 55 is then injected into the groove 54 and cured. After taking out the molded electrode substrate 50 from the mold 53, the part providing the end surface to be brought into sliding contact with a recording sheet is polished to complete an electrostatic recording head 56 having the electrode ends 52a exposed at the end surface of the recording head as shown in FIG. 11.
The exposed ends 52a of the recording electrodes of the electrostatic recording head 56 are demanded to have a high degree of linearity over the whole length L in the longitudinal direction of the recording head 56. Especially, when three or four electrostatic recording heads 56 are arranged in parallel to each other for recording a color image as described above, not only a high degree of uniformity of the thickness in the widthwise direction of each of the electrode substrates 50 but also a high degree of linearity of the row of the exposed electrode ends 52a in each recording head 56 is required so as to prevent appearance of undesirable color nonuniformity or color blur on the recorded color image.
However, because the electrode substrate 50 is made by bonding together two thin sub-substrates 1 and 2 each having a thickness of, for example, 500 .mu.m to 1 mm, wavy deformation tends to occur on the electrode substrate 50 during the molding process using a mold as shown in FIG. 10, and it is difficult to achieve the desired high degree of linearity of the row of the exposed electrode ends 52a.